HEREDITY OF SEX 273 



of chromosomes. An irregularity is accordingly intro- 

 duced into the process of fusion of the chromosomes 

 in pairs, which, as already described, always precedes 

 the formation of the germ-cells with their reduced 

 number of chromosomes. In the case of the male 

 Protenor all the chromosomes fuse in pairs except one, 

 which is, of necessity, left over. This odd chromosome 

 is described as the heterotropic chromosome. The 

 female Protenor has one more chromosome in its 

 somatic cells than the male, thus making up an even 

 number; that is to say, in the female the pair to 

 the odd chromosome of the male is present, so that 

 there are two heterotropic chromosomes, or idio- 

 chromosomes. These fuse and separate in the reducing 

 division, which thus proceeds in the normal manner 

 in this sex. In the male, on the other hand, when 

 the reducing division occurs, the heterotropic chromo- 

 some passes complete into one of the resulting cells. 

 In the second gamete-producing division, every chro- 

 mosome present having divided into two, the products 

 of this division pass into different gametes. These 

 latter divisions are of two kinds, since in one of them 

 the heterotropic chromosome takes part, whilst in the 

 other it is wanting; consequently, two out of the 

 four spermatozoa eventually produced contain the 

 heterotropic chromosome and two do not. (Only one 

 spermatozoon of each kind is shown in Fig. 47.) Thus 

 there is a differentiation of the spermatozoa into two 

 different kinds, and one of these kinds contains a 

 chromosome less than the other. On the other hand, 



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